Dielectric ceramic composition

ABSTRACT

A dielectric ceramic composition comprising a ternary system composition represented by the formula, aBaO.bTiO 2 .cWO 3  (wherein a, b and c are percent molar fractions totaling 100, i.e., a+b+c=100; 17≦a≦21.5; 75≦b≦83 and 0.1≦c≦5) and characterized by an advantageous properties, particularly, high dielectric constant, large Q and small τf. In the compositions, TiO 2  may be partially replaced by ZrO 2  and, optionally, MnO may be also included up to 3 mole percent based on the total molar amount of the foregoing ingredients. The dielectric ceramic compositions are especially, but not exclusively, useful in microwave applications because of the foregoing advantages.

BACKGROUND OF THE INVENTION

The present invention relates to a dielectric ceramic compositionadapted for use in microwave applications which comprises three maincomponents of BaO, TiO₂ and WO₃. More particularly, this inventionrelates to a dielectric ceramic composition especially, but notexclusively, useful for microwave dielectric resonators.

Recently, there have been used dielectric materials having a highdielectric constant, a low dielectric loss and a small temperaturecoefficient of resonance frequency in resonators. Especially, suchmaterials have been now brought into practical use in applications, suchas receivers for a satellite broadcasting, communication devices orequipment, such as automobile telephone, or broadcasting system.

BaO.TiO₂ system dielectric materials have heretofore been known in suchapplications. For example, dielectric materials comprising Ba₂ Ti₉ O₂₀are discussed in detail in U.S. Pat. No. 3,938,064 of H. M. O'Bryan etal, entitled "Devices Using Low Loss Dielectric Material" issued Feb.10, 1976 (Japanese Patent Publication No. 58-20 905). Further, compositeperovskite dielectric materials, typically Ba(Zn_(1/3) Ta_(2/3))O₃, hasbeen described in Japanese Patent Application Laid-Open No. 53-35 454.However, these known materials have the following disadvantages orproblems.

The above Ba₂ Ti₉ O₂₀ dielectric ceramics are required to be treatedwith an acidic solution after calcination or to be treated with anoxygen atmosphere over a long period of time after firing, in order toimprove their dielectric properties. It has been reported in "Ba₂ Ti₉O₂₀ as a Microwave Dielectric Resonator" and "A new BaO.TiO₂ Compoundwith Temperature-Stable High Permittivity", both presented by H. M.O'Bryan et al at American Ceramic Society 76th Annual Meeting that thedielectric ceramics thus obtained have a Q value of 4200 at 10 GHz,where Q is the reciprocal of dielectric loss tanδ, i.e., Q=1/tanδ. Also,O'Bryan et al further describe in the aforementioned U.S. Pat. No.3,938,064 that acid leaching of calcined matrials exhibits anadvantageous effect of reducing the dielectric loss of the final ceramicproducts. For example, there are shown improved Q values of 9,400 forHNO₃ leaching, 9,200 for HCl leaching and 9,600 for H₂ SO₄ leaching incontrast to the Q of 4,900 for unleached material. Reoxidation treatmentat a temperature of 900° to 1400° C. for a period of 10 to 100 hours isrequired in certain materials. However, the resulting Q value isinsufficient for practical use in resonators and a further increased Qvalue has been needed.

On the other hand, in the preparation of the perovskite type ceramicmaterial, large amounts of expensive materials, such as Nb₂ O₅ or Ta₂ O₅are necessitated and will result in an increased cost.

Accordingly, an object of the present invention is to improve theforegoing disadvantages and thereby to provide dielectric ceramicshaving an increased Q value and a small temperature coefficient ofresonance frequency which make them ideal for use in microwave devices.Hereinafter, "temperature coefficient of resonance frequency" isreferred to as ".sup.τ f".

SUMMARY OF THE INVENTION

In view of the foregoing object of the present invention, the presentinvention provides dielectric ceramic compositions mainly composed of aBaO-TiO₂ -WO₃ ternary system and, if necessary, additives of ZrO₂ andMnO may be contained in the compositions.

A first aspect of the present invention resides in a dielectric ceramiccomposition suited for microwave applications which comprises a ternarysystem composition represented by the formula,

    BaO.bTiO.sub.2.cWO.sub.3

(wherein a, b and c are percent molar fractions totaling 100, i.e.,a+b+c=100; 17≦a≦21.5; 75≦b≦83; and 0.1≦c≦5).

In another aspect of the present invention, there is provided a furtherdielectric ceramic composition comprising a ternary system compositionmodified with ZrO₂ and represented by the formula, ##EQU1## (wherein a,b and c are mole percentages totaling 100, i.e., a+b+c=100; 17≦a≦21.5;75≦b≦83; 0.1≦c≦5; and 0≦n<10) and up to 3 mole percent MnO, based on thetotal molar amount of the a, b and c. Throughout the specification, thecomposition represented by the latter formula, ##EQU2## is referred toas ternary system composition, because it comprises basically the samemain three components (i.e., BaO, TiO₂ and WO₃) as the composition ofaBaO.bTiO₂.cWO₃, although a portion of TiO₂ is replaced by a smallamount of ZrO₂.

Heretofore, BaO and TiO₂ components have been both used in large amountsin materials for low-frequency capacitors and are inexpensive. Accordingto the present invention, there can be produced dielectric ceramicshaving a high Q value by adding WO₃ to these components, by a simpleprocess without requiring any extra treatment such as acid treatment orreheating treatment in an oxygen atmosphere, at a low production cost.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, the single figure is a graph showing the relationshipbetween the proportions of WO₃ by mole percent and Q values.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the dielectric ceramic composition of the present invention, thereason why the proportions of BaO and TiO₂ are limited to 17 to 21.5mole percent and 75 to 83 mole percent, respectively, is that when thepercent molar fractions of these components fall outside the abovespecified ranges, .sup.τ f value is too large or Q value is too small.

When the ratio of WO₃ is less than 0.1 mole percent, Q values becomesmall, while WO₃ in a molar ratio beyond 5 mole percent results in anunacceptably increased .sup.τ f value and reduced Q value.

In a relatively small content range of TiO₂ (i.e., 75≦b≦81.8) within thepresent invention, the WO₃ component exhibits a .sup.τ f reducingeffect. This effect can be obtained when WO₃ is present in a smallpercentage and a large proportion of WO₃ results in a large .sup.τ f.Consequently, the proportion of WO₃ should be practically limited to amaximum of 5 mole percent.

Substitution of ZrO₂ for TiO₂ leads to a reduction in .sup.τ f over theentire range of the specified composition range and thus makes it easyto obtain the desired levels of .sup.τ f. The reason why the amount ofthis replacement by ZrO₂ is limited below 10 mole percent is that whenZrO₂ is contained in amounts of 10 mole percent or more, Q becomes toosmall.

MnO promotes the densification of materials during the process of firingand thereby improves the Q of the final products. Further, .sup.τ f isslightly variable with the amount of MnO. Since an excess content of MnObeyond 3 mole percent reduces Q values to an unfavorable level, MnOshould be limited to the content range of 3 mole percent or less.

The dielectric ceramic composition of the present invention can beobtained by a usually practiced process in the art without requiring anyextra special processing step. More specifically, as source materials,compounds, such as MnO₂ or Mn₂ O₃ having valencies different from thevalencies in the resulting fired ceramic products may be used and,besides oxides, carbonates, such as BaCO₃ or MnCO₃, may be also used. Inother words, any compound may be generally used as long as it caneventually give fired oxide products.

In the preparation of the dielectric ceramic composition, sourcematerials are weighed depending on the desired composition and are wetblended thoroughly with water in a polyethylene pot containing aluminaballs. The resulting mixture is dried and is calcined in an aluminacrucible at temperatures of 1000° to 1200° C. for a period of 1 to 6hours. The calcination operation is conducted in an oxidizingatmosphere, such as air or oxygen. After this calcination, the calcinedmixture is wet milled with pure water in a polyethylene pot containingalumina balls, dried and then shaped by a compression molding. Theshaped material is fired on a setter made of zirconia or platinum at atemperature of 1350° to 1420° C. for a period of 2 to 6 hours in anoxygen atmosphere.

Now, the present invention will be described in detail with reference tothe following examples hereinafter. Examples:

Various dielectric ceramic compositions were prepared in accordance withthe manner previously described, using BaCO₃, TiO₂, WO₃, ZrO₂ and MnCO₃as starting materials and were tested for dielectric constant, Q and.sup.τ f in the temperature range of -25° C. to +75° C., at frequenciesof 9.5 to 10.5 GHz .

The proportion of each component and test data are given in Table belowand Sample Nos. 27 to 33 are shown for comparison purposes.

                  TABLE                                                           ______________________________________                                                           Dielectric Properties                                                         at 9.5 to 10.5 GHz                                                                  Dielec-                                              Sam-                     tric                                                 ple  Composition (Mole Percent)                                                                        Con-         τ.sub.f                             No.  BaO    TiO.sub.2                                                                            WO.sub.3                                                                           ZrO.sub.2                                                                           MnO  stant Q    ppm/°C.                  ______________________________________                                         1   17.8   80.2   2    --    --   41    5600 45                               2   17.3   77.7   5    --    --   42    5200 102                              3   18.1   81.4   0.5  --    --   39.3  5200 11                               4   18.1   81.4   0.5  --    0.1  39.5  6000 12                               5   18.1   81.4   0.5  --    0.5  39.7  5000 16                               6   18.1   81.4   0.5  --    1    40    5300 18                               7   18.1   81.4   0.5  --    2    38.6  5100 20                               8   18.5   81.65  0.2  --    0.2  39.3  5800 6                                9   18.1   81.4   0.5  --    0.2  39.7  5700 15                              10   18.1   81.4   0.5  --    0.1  39.5  6000 12                              11   18     81     1    --    0.2  40.0  5600 27                              12   17.8   80.2   2    --    0.2  40.4  5600 49                              13   17.3   77.7   5    --    0.2  42.1  5300 106                             14   21.1   77.9   1    --    0.2  35.8  5700 23                              15   20.3   79.2   0.5  --    0.2  37.3  5600 21                              16   19.9   79.6   0.5  --    0.2  37.7  5600 16                              17   19.8   79.2   1    --    0.2  37.1  5600 10                              18   19     76     5    --    0.2  37.5  5400 55                              19   19.1   80.4   0.5  --    0.2  38.4  5300 8                               20   18.8   80.7   0.5  --    0.2  38.7  5300 5                               21   18.4   81.1   0.5  --    0.2  39.3  5400 6                               22   17.8   81.7   0.5  --    0.2  40.2  5200 23                              23   17.5   82     0.5  --    0.2  40.3  5200 31                              24   18.1   80.4   0.5  1     0.2  39.7  5700 14                              25   18.1   78.9   0.5  2.5   0.2  39.4  5500 13                              26   18.1   76.4   0.5  5     0.2  38.5  5300 8                               27   18.1   71.4   0.5  10    0.2  35.4  2000 -15                             28   18.1   66.4   0.5  15    0.2  30.4  1500 5                               29   18.2   81.8   --   --    0.2  37.1  2600 2                               30   16.8   75.7   7.5  --    0.2  44.0  3000 un-                                                                           measured                        31   22     77     1    --    0.2  33.4   400 7                               32   16.6   82.9   0.5  --    0.2  42.1  4200 56                              33   18.1   81.4   0.5  --    4    38.7  1600 11                              ______________________________________                                    

Sample Nos. 1 to 3 are compositions made up of BaO, TiO₂ and WO₃ andSample Nos. 4 to 7 further contain MnO in addition to these threeingredients. As readily seen from the test results in the Table, thesesamples all exhibit small dielectric loss and small .sup.τ f propertiesand the MnO addition provides dielectric ceramics of finer texture andhigh density, thereby resulting in a high dielectric constant. Further,it has been found that the MnO addition has an effect in improving Q.However the MnO addition should be limited up to 3 mole percent, since Qis reduced with an increase in the addition amount of MnO and, forexample, as will be apparent from Sample No. 33, an excess MnO additionabove 3 mole percent results in an excessively reduced Q.

Sample Nos. 8 to 13, 29 and 30 are the composition containing BaO andTiO₂ in a molar ratio of BaO/TiO₂ =1/4.5 in which WO₃ in the range of 0to 7.5 mole percent is contained with MnO in the range of 0.1 to 0.2mole percent in these samples. With respect to the proportion of WO₃,the invention ceramics containing WO₃ not exceeding 5 mole percentpossess greatly improved Q values of 5300 to 6000, as compared with Q ofabout 2600 of MnO-free ceramics. However, when the content of WO₃ exceed5 mole percent, .sup.τ f is increased excessively and Q is unfavorablyreduced. The drawing illustrates this relationship of molar percentagesof WO₃ and Q values at 10 GHz.

The compositions of Sample Nos. 14 to 23 and 31 to 32 consist of 16.6 to22 mole percent BaO, 77 to 82.9 mole percent of TiO₂ and 0.5 to 5 molepercent WO₃. Particularly, in comparison of the compositions of SampleNos. 16 and 17 containing TiO₂ less than 81.8 mole percent, i.e., 79.2mole percent and 79.6 mole percent of TiO₂, and having a BaO:TiO₂ molarratio of 1:4, it can be known that the composition containing 1.0 molepercent of WO₃ has a smaller .sup.τ f value than the composition with0.5 mole percent of WO₃. As shown in such experimental data, in theinvention ceramic compositions with a relatively low content of TiO₂,WO₃ has an effect of reducing .sup.τ f and makes possible the attainmentof small .sup.τ f over a wide composition range. In the compositionscontaining BaO and TiO₂ both in amounts outside the ranges of thepresent invention, for example as in Sample Nos. 31 and 32, it has beenfound that .sup.τ f becomes too large and Q becomes too small.

The compositions of Sample Nos. 24 to 28 comprise 18.1 mole percent ofBaO, 81.4 mole percent of TiO₂ and 0.5 mole percent of WO₃ in which TiO₂is partially replaced by ZrO₂ within the range of 1 to 15 mole percent.In these compositions, 0.2 mole percent of MnO was added. It is apparentthat .sup.τ f is decreased with an increase in the replacement by ZrO₂and when the replaced amount by ZrO₂ reaches 10 mole percent or greaterlevels, Q is too small.

As described above, according to the present invention, there can beobtained dielectric ceramic compositions having an advantageouscombination of properties, particularly high dielectric constant, small.sup.τ f and large Q, in a usual processing manner without requiringextra special step. .sup.τ f is adjusted by changing the proportion ofeach component. Although the dielectric ceramic compositions of thepresent invention are described and illustrated in reference withmicrowave applications, they also give good utility intemperature-compensating capacitors or other applications because oftheir good electrical properties in a low frequency region.

What is claimed is:
 1. A dielectric ceramic composition which comprisesa ternary system composition represented by the formula,aBaO.bTiO₂.cWO₃wherein a, b and c are percent molar fractions and a+b+c=100; 17≦a≦21.5;75≦b≦83; and 0.1≦c≦5.
 2. A dielectric ceramic composition whichcomprises a ternary system composition represented by the formula,##EQU3## wherein a, b and c are percent molar fractions and a+b+c=100;17≦a≦21.5; 75≦b≦83; 0.1≦c≦5 and 0≦n<10.
 3. A dielectric ceramiccomposition as claimed in claim 1, containing up to 3 mole percent ofMnO, based on the total molar amount of a, b and c.
 4. A dielectricceramic composition as claimed in claim 2, containing up to 3 molepercent of MnO, based on the total molar amount of a, b and c.